Hematopoietic stem cells are a rare population of cells which have a unique biological profile. This population can be used for cellular therapy of hematological malignancy, bone marrow failure disorders, as well as gene therapy. for more than a decade the CD34 antigen has been used to help isolate this rare population in humans. Transplantation of CD34 positive cells in patients has resulted in prompt engraftment of donor cells. Recently, through the development of antibodies to the CD34 antigen on mouse tissues, it has become possible to study the stem cell biology (i.e., proliferation, differentiation, survival, and self- renewal) using a transplant strategy to determine the importance of CD34 in identifying stem cells. Most stem cell biologists will agree that the stem cell pool is heterogenous with the earliest (highest quality) stem cell having potentials for proliferation, differentiation, survival, and self-renewal but as the stem cell matures these potentials are lost. It becomes critically important in diseases with stem cell intrinsic defects to use the highest quality stem cell for cellular therapy. Recently, conflicting reports have appeared which have made it more difficult to assess functional significance of the CD34 antigen. Our first specific aim will examine the role of CD34 for self-renewal, proliferation, and differentiation. This will involve transplantation of CD34 null stem cells into normal mice and normal cells into null mice to test the competence of these cells in a transplantation protocol. We will also examine the recovery of endogenous stem cells to either radiation or 5-fluorouracil exposure to assess the endogenous competence of CD34 null stem cells. Specific aim 2 will study the compensatory mechanisms that may be at play in allowing CD34 null mice to survive to adulthood with mildly affected hematopoietic phenotypes. Finally, we will study the homing potential of stem cells from CD34 null mice to engraft normal recipients using a recently developed homing assay in which we will tag stem cells in vivo. We expect that the results of our studies will be directly applicable to treatment of hematological disorders, genetic diseases, and malignancy.